1. Field of the Invention
The present invention relates to an anti-reflection film and an optical system using the same and, more particularly, to an anti-reflection film, which attains anti-reflection characteristics for the ultraviolet range having a wavelength of 250 nm or less by stacking a plurality of predetermined refractive index layers on the surface of an optical element substrate such as a fluorite substrate, synthetic silica glass substrate, or the like, and is effectively applied to, e.g., various types of optical systems used in the manufacture of semiconductor devices.
2. Related Background Art
As a conventional anti-reflection film for ultraviolet rays, an anti-reflection film formed by alternately stacking a plurality of high-refractive index films including an Al.sub.2 O.sub.3 film and a plurality of low-refractive index films including an SiO.sub.2 film on the surface of a transparent substrate has been proposed by, e.g., Japanese Laid-Open Patent Application No. 7-218701.
Also, an anti-reflection film using a fluoride film has been proposed by, e.g., Japanese Laid-Open Patent Application Nos. 7-244205 and 7-244217.
The anti-reflection film using the Al.sub.2 O.sub.3 and SiO.sub.2 films and proposed by Japanese Laid-Open Patent Application No. 7-218701 as the anti-reflection film for ultraviolet rays having a wavelength of 250 nm or less tends to have a relatively narrow anti-reflection bandwidth (wavelength width).
In general, when the anti-reflection bandwidth is narrow, the obtained wavelength width may often vary due to errors in film thickness control during fabrication. When the anti-reflection film is formed on the curved surface of, e.g., a lens, the optical characteristics such as reflectivity, transmittance, and the like often vary considerably depending on different angles of incidence of light.
The anti-reflection film using the fluoride film and proposed by Japanese Laid-Open Patent Application Nos. 7-244205 and 7-244217 can hardly attain reflectivity as low as 0.2% or less in the assumed wavelength range, and the environmental resistance of the fluoride film is inferior to that of oxides (Al.sub.2 O.sub.3, SiO.sub.2).